I can't remember the name of it either. I remember reading one where he cheats on her and her mating mark starts turning black but that's all I remember of it. I tried looking for it but I couldn't find it.
The only other story that is somewhat similar to the one you are looking for is Broken Promises by Oroyukae, but I don't think it is the one your looking for because I don't remember Sess having a child with a full blooded demoness.
She had believed all he had said , whispered at the most intimate of moments in their life together. No, he hadn't lied to her, he couldn't do that . He just repeatedly made promise after promise that he found hard to keep. [ FIN ]